The present invention generally relates to vehicle braking and, more particularly, to an adaptive brake control system that compensates for braking variations to provide consistent brake pedal feel to the vehicle operator.
Conventional friction braking systems installed on automobiles generally employ a brake pedal assembly having a brake pedal coupled to a master cylinder for providing pressurized hydraulic brake fluid to apply friction braking at the wheels of the vehicle. The brake pedal is actuated (depressed) by the vehicle operator to demand an amount of friction braking. Friction braking systems are generally configured to initially provide a consistent brake pedal feel to the vehicle operator such that the amount of friction braking achieved is substantially proportional to the amount of force applied to the brake pedal by the vehicle operator. However, it is generally known that a number of factors may lead to inconsistent brake pedal response. For example, wear on the friction brake pads, brake fade, variations from one brake pedal to another brake pedal in different vehicles, and temperature can influence the brake pedal response, and thus may provide inconsistent friction braking over time. As a consequence, the vehicle operator may experience inconsistent variations in brake pedal feel.
The inconsistent brake pedal response generally experienced with friction braking can become more significant when combined with regenerative braking, commonly found on electric and hybrid vehicles. Regenerative braking systems are typically combined with friction braking systems to convert kinetic energy to electrical energy for storage in the vehicle battery. Regenerative braking is typically powered by an electric motor, which acts as a generator during braking, coupled to the powertrain to convert the kinetic energy to electrical energy. Regenerative braking may also be performed with energy storage technologies that convert the kinetic energy of a vehicle into an alternate form of energy. Concepts exist for recovering vehicle kinetic energy and storing it in the form of pressurized hydraulic fluid, rotating flywheels, and magnetic fields maintained by super conducting coils. The proportional amounts of regenerative braking and friction braking is typically determined by the brake control system, and generally varies depending on a number of factors including vehicle speed, battery charge, and the amount of braking force commanded. As the vehicle brake control system changes the proportional amounts of regenerative braking and friction braking, the inconsistent brake pedal response becomes more significant, since under certain circumstances the proportional amounts of friction and regenerative braking may change quickly, resulting in inconsistent brake pedal response. Thus, the resultant inconsistent brake pedal feel becomes more noticeable to the vehicle operator.
Accordingly, it is desirable to provide for a brake control system in a vehicle that adapts and compensates for inconsistent brake pedal response in friction braking systems. Additionally, it is desirable to provide for a vehicle brake pedal control system for controlling both friction braking and regenerative braking, which compensates for inconsistent brake pedal response experienced in the vehicle braking, so as to provide more consistent brake pedal feel to the vehicle operator.
In accordance with the teachings of the present invention, a vehicle brake system is provided which compensates for brake pedal feel variation so as to provide for enhanced braking feel to the vehicle operator. The brake system includes a brake command input indicative of braking demand requested by a vehicle operator, a friction brake actuator for generating braking force to be applied to one or more brakes on a vehicle, a sensor for sensing kinematic acceleration of the vehicle, and a database storing target accelerations as a function of the brake command input, and further storing a brake parameter that defines a relationship between the brake command input and the friction braking. The vehicle brake system further includes a controller for controlling the friction brake actuator based on the brake demand input and the brake parameter. The controller determines brake acceleration caused by the braking as a function of the sensed kinematic acceleration and determines a target acceleration from the database based on the brake demand input. The controller determines a brake acceleration error as the difference between the target acceleration and the brake acceleration. The controller further adjusts the parameter to reduce the brake acceleration error by adjusting the braking generated by the friction brake actuator so as to compensate for brake variation. According to one aspect of the present invention, the brake actuator includes friction braking and regenerative braking, wherein the amount of friction braking is adjusted to compensate for brake variation.